Abstract: BackgroundPlant-derived residue carbon (PRC) is a core component of soil organic carbon (SOC), and its transformation directly determines the stability of farmland carbon pools. As a carbon-rich material produced by rice straw pyrolysis at 800°C (with a carbon content of 73%), biochar is widely recognized for its potential to enhance soil carbon sequestration. However, uncertainties remain regarding how biochar regulates PRC dynamics-especially the role of application dose and frequency, which are critical for guiding practical farmland carbon management in specific regions like the Yellow River Delta (with saline-alkali soil, adopts a rotation system of wheat and maize) but have been less explored in previous studies.Methods To address this knowledge gap, a four-year field experiment was conducted at the Dongying Base of Shandong Academy of Agricultural Sciences in the Yellow River Delta. The experiment aimed to investigate the effects of biochar application on SOC, lignin phenols (a key biomarker for PRC), lignin monomers (vanillyl-type, V; syringyl-type, S; cinnamyl-type, C), and lignin degradation indices. Ten treatments were designed, including three biochar doses (4, 8, 12 t/ha), three application frequencies (annual, biennial, one-time application), and a no-biochar control (C0). Each treatment had three replicates, and soil samples from the 0-20 cm layer were collected at the wheat harvest stage in 2024 (the 4th year of the experiment) to analyze the target parameters. Results Results showed that all biochar treatments significantly increased SOC content by 15.24%-52.17% compared to the control (C0). However, this SOC accumulation was primarily driven by the direct input of stable carbon from biochar rather than the universal promotion of PRC transformation. A distinct dose-frequency specificity was observed in the regulation of lignin phenols: only the treatments of 8 t/ha annual application (C2) and 4 t/ha biennial application (C4) significantly increased total lignin phenol content by 30.15% and 36.60%, respectively. These two optimal treatments also exhibited differential promotion effects on lignin monomers: V-monomers increased by 16.39%-18.42%, S-monomers by 33.09%-37.04%, and C-monomers by 80.06%-146.10%, which collectively indicated enhanced PRC retention in the soil. In contrast, the high-dose biennial application (12 t/ha, C6) and all one-time application treatments (C7-C9) significantly elevated the acid/aldehyde ratios of V-monomers (20.38%-40.52%) and S-monomers (10.90%-27.83%). These increases in degradation indices clearly signaled accelerated lignin phenol decomposition, which ultimately reduced PRC retention. Correlation analysis further confirmed that excessive lignin degradation-indicated by high S-monomer acid/aldehyde ratios-was negatively associated with total lignin phenols and C-monomers (P<0.05), emphasizing the risk of PRC loss under improper biochar application regimes.ConclusionIn conclusion, biochar regulates PRC dynamics in the farmland soils of the Yellow River Delta through distinct dose-frequency effects. Specifically, the treatments of 8 t/ha annual biochar application and 4 t/ha biennial biochar application successfully achieved a balanced “degradation-accumulation” state of lignin phenols, thereby favoring PRC sequestration. These findings not only clarify the interaction mechanism between biochar and PRC in warm-temperate agricultural ecosystems of the Yellow River Delta but also provide a critical theoretical basis for optimizing biochar application strategies to synergistically enhance SOC stocks and PRC stability in this region.